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Abstract
Context. Collapsing magnetic traps (CMTs) have been suggested as one possible mechanism responsible for the acceleration of highenergy particles during solar flares. An important question regarding the CMT acceleration mechanism is which particle orbits escape and which are trapped during the time evolution of a CMT. While some models predict the escape of the majority of particle orbits, other more sophisticated CMT models show that, in particular, the highestenergy particles remain trapped at all times. The exact prediction is not straightforward because both the loss cone angle and the particle orbit pitch angle evolve in time in a CMT. Aims. Our aim is to gain a better understanding of the conditions leading to either particle orbit escape or trapping in CMTs. Methods. We present a detailed investigation of the time evolution of particle orbit pitch angles in the CMT model of Giuliani and collaborators and compare this with the time evolution of the loss cone angle. The nonrelativistic guiding centre approximation is used to calculate the particle orbits. We also use simplified models to corroborate the findings of the particle orbit calculations. Results. We find that there is a critical initial pitch angle for each field line of a CMT that divides trapped and escaping particle orbits. This critical initial pitch angle is greater than the initial loss cone angle, but smaller than the asymptotic (final) loss cone angle for that field line. As the final loss cone angle in CMTs is larger than the initial loss cone angle, particle orbits with pitch angles that cross into the loss cone during their time evolution will escape whereas all other particle orbits are trapped. We find that in realistic CMT models, Fermi acceleration will only dominate in the initial phase of the CMT evolution and, in this case, can reduce the pitch angle, but that betatron acceleration will dominate for later stages of the CMT evolution leading to a systematic increase of the pitch angle. Whether a particle escapes or remains trapped depends critically on the relative importance of the two acceleration mechanisms, which cannot be decoupled in more sophisticated CMT models.
Original language  English 

Article number  A73 
Number of pages  12 
Journal  Astronomy & Astrophysics 
Volume  563 
Early online date  12 Mar 2014 
DOIs  
Publication status  Published  Mar 2014 
Keywords
 Sun: corona
 Sun: activity
 Sun: flares
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Dive into the research topics of 'Loss cone evolution and particle escape in collapsing magnetic trap models in solar flares'. Together they form a unique fingerprint.Projects
 1 Finished

Plasma Theory: Solar and Magnetospheric Plasma Theory
Hood, A. W. (PI), Mackay, D. H. (CoI), Neukirch, T. (CoI), Parnell, C. E. (CoI), Priest, E. (CoI), Archontis, V. (Researcher), Cargill, P. (Researcher), De Moortel, I. (Researcher) & Wright, A. N. (Researcher)
Science & Technology Facilities Council
1/04/13 → 31/03/16
Project: Standard